Research conducted over the last decade has provided a wealth of new information on the molecular mechanisms utilized by DNA tumour viruses. Studies of tumour viruses have led to important insights into the functions of viral proteins and into the regulation of normal cellular proliferation. DNA tumour viruses can stimulate growth factor signaling pathways, alter gene transcription, and inactivate growth suppressor proteins. Members of the polyoma and adenovirus families express proteins that interact with protein serine/threonine phosphatase 2A (PP2A). In the case of SV40 virus, this interaction plays an accessory role in transformation of most cells, while it is essential for transformation of some cell types. The topics discussed in this review include the interactions of these viral proteins with PP2A, the effects of these interactions on phosphatase activity, and how these interactions alter cellular signal transduction pathways.